In general, a cascade boiler system has a capacity of a middle or large size boiler by connecting a plurality of boilers in parallel. While a control method thereof is more difficult than the case in which a middle- or large-sized boiler is used, the heating can be controlled according to circumstances, and a heating capacity can be increased according to necessity.
The cascade boiler system generally uses a configuration in which a hydro-separator is provided between a boiler side to which a plurality of boilers are connected in parallel and a load side serving as an indoor pipeline section. This is to prevent occurrence of a phenomenon of lack of a supply flow rate of hot water in which a flow rate of the boiler side is lower than a necessary flow rate of the load side when only some of the plurality of boilers are operated according to circumstances.
The hydro-separator functions to mix returned water returned to the boiler side from the load side with hot water to supplement the supply flow rate of the hot water when the flow rate of the hot water supplied from the boiler side to the load side is low.
A configuration and an action of the hydro-separator are disclosed in detail in Korean Patent No. 10-1172215 (entitled “Cascade system, method for controlling the same, and heating-dedicated boiler constituting the same,” registered on Aug. 1, 2012, see FIGS. 1 to 3). A control method when the hydro-separator of the related art is used and problems of the control method will be described as follows.
FIG. 1 is a systemic configuration view for describing a method for controlling a cascade boiler system of the related art.
Referring to FIG. 1, the cascade boiler system includes a primary side 10 to which a plurality of boilers 11, 12, 13, 14 and 15 are connected in parallel, a secondary side 20 having loads 21 and 22, and a hydro-separator 30 configured to connect the primary side 10 and the secondary side 20 and correct a supply flow rate.
In such a configuration, a heating temperature is set with reference to a supply water temperature T3 of the secondary side, three boilers 13, 14 and 15 of the primary side 10 are operated, and a sum of flow rates of pumps provided at the boilers 13, 14 and 15 becomes a flow rate F1 of the primary side 10.
Here, while a normal operation of the system may be performed if the flow rate F1 of the primary side 10 is equal to a flow rate F2 of the secondary side 20, when the flow rate F1 of the primary side 10 is smaller than the flow rate F2 of the secondary side 20, a supplementary flow rate F3 of the hydro-separator 30 that is a portion of a flow rate of water returned from the secondary side 20 to the primary side 10 is added to the supply flow rate F1 of the primary side 10 to become the flow rate F2 of the secondary side 20 in the hydro-separator 30.
Here, since the flow rate F2 of the secondary side 20 is added by the supply water heated by the boilers 13, 14 and 15 of the primary side 10 and the supplementary flow rate F3 of the returned water having a temperature decreased by passing the loads 21 and 22 of the secondary side 20, the supply water temperature T3 of the secondary side 20 is lower than a target temperature Tt.
In this state, if it is normal, the boilers 11 and 12 that are not in operation should be operated, and a process of determining a supply flow rate again as described above is repeated in a state in which the boiler 12 is operated, thereby determining whether the supply water temperature T3 of the secondary side 20 is equal to a target temperature Tt. While the above-mentioned determination procedure is repeated to adjust the supply water temperature T3 of the secondary side 20 to be controlled to maximally approximate the target temperature Tt, the procedure is time-consuming to cause a consumer's dissatisfaction and decrease the consumer's trust with respect to the boiler system.
In addition, since the primary side 10 has a large temperature difference between a returned water temperature T2 and the supply water temperature T1, the supply water temperature T3 of the secondary side 20 cannot be easily controlled in a state in which the boiler 12 is operated, i.e., the temperature T3 may be higher than the target temperature Tt.
In order to solve the problems, while a method of increasing a circulation flow rate of the boilers 11 to 15 by adding a pump to the outside may be used, production cost and maintenance cost of the cascade boiler system are increased.
In addition, since the flow rates are detected at the respective positions to control the boiler in the related art, a flowmeter 40 is needed to increase the cost of the cascade boiler system.